1. Field of the Invention
The present invention relates to a toner bearing member, a developing device having the toner bearing member, and an image forming apparatus equipped with the developing device.
2. Description of the Related Art
A developing device employing an electrophotographic process is used in an image forming apparatus, such as a photocopier, printer, and facsimile Among such developing devices, a non-contact type developing device has attracted attention where the non-contact type developing device performs developing without contacting a toner bearing member, which transport a developer (a toner), with a photoconductor on which a latent electrostatic image is formed. As for the non-contact developing system, known are developing methods utilizing a powder round system, a jumping system, or an electric field curtain system.
The jumping system is a system where the toner is made to jump between the photoconductor to the toner hearing member, and requires an applied voltage that is equal to or stronger than the adhesion between the toner and the surface of the toner bearing member.
The electric field curtain system is to supply a toner to a latent electrostatic image by applying an alternative electric field to a plurality of electrode provided and aligned with a certain pitch in a toner bearing member, and making the previously charged toner hop by the electric field curtain formed by alternating a non-uniform electric field formed on the surface of the toner bearing member. Since the toner hops at the surface of the toner bearing member, the adhesion between the toner and the toner bearing member becomes almost nil. A force for separating the toner from the surface of the toner hearing member for developing is not necessary, and therefore, the toner can be sufficiently transported to the side of the photoconductor with application of a low voltage.
For example, in the case where a toner bearing member, in which a plurality of electrode are covered with a surface protective layer formed of an insulating material, is used in a developing device of an electric field curtain system, an electric charge of the toner does not leak to the electrode, and hopping failures, which may occur because of loss of electric charge of the toner, do not occur (see Japanese Patent Application Laid-Open (JP-A) No. 03-21967).
Moreover, proposed is to form a surface of a toner bearing member with a material accelerating frictional electrification to a regular charge polarity of the toner, for supplying a toner to a surface of the toner bearing member without frictional electrification of the toner in advance, and making the toner hop with an alternating electric field to thereby charge the toner (see JP-A No. 2007-133388).
Moreover, proposed are a toner bearing member using bisphenol Z polycarbonate in a surface layer thereof, a developing device equipped with the toner bearing member, and an image forming apparatus equipped with the toner bearing member (see JP-A No. 2010-281859). In this proposal, however, the surface layer of the toner bearing member is abraded and the electrode is exposed, as a number of fed sheets increases, which causes leaking. The toner bearing member causing leaking leaves a leak mark, and at a surface of which toner hopping cannot be formed regularly. If an image is output in such a state, a defected image having a defected portion in which a toner density is low is formed.
Accordingly, it is desired to form a surface layer of a toner bearing member with a material having abrasion resistance in order to stably perform image formation over a long period. Moreover, the material of the surface layer is also required to have an appropriate toner charging ability. Although a surface layer is formed with an insulating material or a material accelerating frictional electrification to the regular charge polarity of the toner, a strong electrostatic attractive force is generated between the toner and the surface layer of the toner bearing member, if electrification of the toner and that of the surface of the toner are too strong. This attractive force overcomes the powder of the toner to hop by receiving the electric field from the electrode inside the toner bearing member, and therefore the toner continues to adhere to the surface of the toner bearing member, and does not hop. The toner cannot be charged unless the toner is sufficiently hopped, and moreover a toner cloud is not formed, which lead to formation of a defected image.
Here, “toner cloud” means that the toner is floated in the air in the form of a mist by intermittently repeating hopping of the toner.
Even when a toner cloud is formed and a regular image can be output at an initial stage, a balance between an attractive force of the toner to the toner bearing member and toner hopping is easily destroyed, as a number of the fed sheets increases, due to a change in the electric field from the electrode inside the toner bearing member caused by abrasion of the surface layer of the toner bearing member, a change in electrostatic charge of the toner caused by a change in an amount of the toner transported to the toner bearing member as a result of a change in the surface condition (roughness) of the surface layer, and a change in tackiness of the toner bearing member to the toner (attractive force of the toner bearing member to the toner). As a result, the toner is adhered to the surface of the toner bearing member, and therefore the toner does not sufficiently hop even when the toner receives the electric field from the electrode inside the toner bearing member, which reduces output image density. Therefore, it is extremely difficult to output regular images after feeding, as output image density is low.
Accordingly, there is currently a need for a toner bearing member whose surface layer is formed of a material having a toner charging ability capable of providing a toner with electric charge that sufficiently induce toner hopping, and having high abrasion resistance, and which can prevent output of a defected image by achieving the aforementioned properties, as well as needs for a developing device having the toner bearing member, and an image forming apparatus equipped with the developing device.